A Non-Weight Bearing Method for Measuring Hip Abduction Strength Overestimates Hip Abductor Muscle Fatigue During One-Leg Stance.

ABSTRACT

OCCUPATIONAL APPLICATIONSAssessing workers' strength capacities is a common practice prior to return to work following injury or illness, or assessing capabilities for strenuous jobs. Because it requires 50% or more of maximum strength capacity, hip abductor muscle strength is a strong predictor of both middle- and older-aged individuals' ability to reliably balance on one leg and of their risk of falls. Our results suggest subjects were able to augment their hip abductor moment during unipedal weight stance via gluteus maximus activity. Weight-bearing hip abduction strength measures are important for assessing worker capacity for jobs requiring reliable unipedal balance whether during lateral loading, while walking in gusty winds, on slippery footing or resisting lateral deck movements on board ship or train. Measurements of hip abductor strength should be made in a full unipedal weight bearing posture; non-weight-bearing measurements significantly underpredicted hip abductor strength as well as endurance.

Background: Unipedal stance time is a significant predictor of fall risk in middle- and older-aged individuals. Because muscle fatigue reduces muscle strength over a sustained contraction, muscle endurance time could limit maximum unipedal stance time. Purpose: We tested the hypothesis that hip abduction muscle endurance time, which depends on muscle strength, limits maximum unipedal stance time. Methods: We recruited 18 healthy young (10 females) and 17 older (10 females) adults who could stand on one leg for at least 10 s. Using a non-weight bearing test, we measured the left hip isometric maximum abduction strength and endurance time at 50% of that strength. We also measured subjects' maximum unipedal stance time, while force plate and full-body 3D motion capture signals were recorded. Subject-specific inverse dynamics biomechanical models were used to calculate the hip abduction moment required to stand on one leg. Results: Unipedal balance required a mean (SD) of 59 (19) % and 118 (53) % of the young and old adult hip abduction muscle strengths, respectively, which were consistent with published values. Mean (SD) hip muscle endurance times at 50% of abduction MVC were 63 (62) sec. and 147 (107) sec. for young and older adults, respectively. However, contrary to our hypothesis (above), the endurance time at 50% MVC failed to predict an upper limit for the maximum unipedal stance time. Conclusions: Our results show that standing on one leg requires over 50% of non-weight bearing hip abductor strength when young, and full strength when older. Because all younger individuals and four (out of 17) older individuals could balance on one leg longer than predicted during weight-bearing the gluteus maximus, a hip extensor, likely tensioned the iliotibial band to counteract abductor muscle fatigue with time. So, measurements of hip abduction strength should be made in the weight-bearing posture when middle- and older-aged workers' balance capacity is being assessed whether in a return-to-work assessment setting or evaluating workers for jobs requiring significant lateral loading, walking in gusty winds or on slippery surfaces, or on boat or train decks prone to sudden movements.